 Parameters
of the transistors used in high frequency |
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Created it, 05/10/15
Update it, 06/01/16
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SEMICONDUCTORS 6 “13th part”
7. - PARAMETERS “r”
Other parameters, used generally for the transistors low frequency and low power, are indicated by the symbol R because they all are homogeneous with resistances and their values are thus expressed in W or kW.
This system is consisted by three parameters
expressed in W and the pure
number or coefficient 
or called hfb. They are indicated by the
symbols re, rb and rc
because they respectively represent the values of internal resistances of the
transistor for the transmitter, the base and the collector. Since a separation
of three internal resistances is not possible physically, but only theoretically,
it is included/understood whereas these parameters cannot be measured in
practice like that was made for the parameters h ; one can only calculate
their value while leaving, for example, of the parameters h.
One of the advantages which the parameters r offer is to allow obtaining simpler formulas when designing of amplifiers and circuits in general.
Another advantage is due to the fact that these parameters keep the same value in assembly bases common and thus that contrary to the parameters h, it is not necessary to differentiate the parameters bases common parameters common transmitter.
Sometimes, in the handbooks of the manufacturers,
only are indicated the values of the parameters r
and of the coefficient hfb or ,
whereas it can be interesting to know the values of the parameters h
relating to the assembly bases common and common transmitter.
The latter can be easily deduced from the first by using the following formulas in which the values of rb and re must be expressed in W and the value rc in kW ; in this way, the parameters hib and hie are expressed in W and the parameters hob and hoe in µA / V, i.e. in µS (microsiemens).
For the assembly bases common, one a :
hib = re + rb x
(1 - )
hfb =
hrb = rb / (1 000 x rc)
hob = 1 000 / rc
For the common transmitting assembly, one a :
hie = rb + re x
(1 - )
hfe =
/ (1 - )
hre = re / (1
000 x rc) x (1 - )
hoe = 1 000 /
rc x (1 - )
The parameters r are obtained starting from the parameters h relating to the common transmitting assembly by the following relations :
rb = hie - (hre / hoe) x (1 + hfe)
rc = (1 + hfe) / hoe
re = hre / hoe
= hfe / (1 + hfe)
Having obtained the parameters r, one can calculate the parameters hb with the formulas seen previously ; thus, in two stages of calculation one can obtain the parameters hb relating to the assembly bases common which are not always available, on the basis of those of the transmitting assembly common he.
For example :
For hie = 1,1 kW ; hfe = 50 ; hre = 0,00025 ; hoe = 25 µS.
One obtains the parameters r
following : rb = 590 W
; rc = 2,04 MW
; re = 10 W
;
= 0,98 and the parameters hb are
thus : hib = 21,6 W
; hfb = 0,98 ; hrb = 0,00029
; hob = 0,49 µS.
Lastly, one can seek the parameters hc relating to the common collecting assembly. They are calculated easily starting from the parameters he by means of the following formulas :
hic = hie
hrc = 1 - hre
hfc = 1 + hfe
hoc = hoe
In the preceding case, one obtains : hic = 1,1 kW ; hfc = 51 ; hrc = 0,99975 ; hoc = 25 µS.
It should be noted that in this assembly where the profit in tension is appreciably equal to 1, the coefficient of reaction hrc is him also almost equal to 1 and this independently of the other parameters.
To conclude, figure 9 presents the curves which give the parameters he of the transistor to the low frequencies and weak signals (low power) standard BC 108, according to the collector current and for two values of the tension of collector (VCE = 5 V and VCE = 10 V).
8. - PARAMETERS
OF THE TRANSISTORS USED IN HIGH FREQUENCY
The four parameters seen previously are not sufficient to determine in a precise way the characteristics of a transistor intended to function at high frequencies. Indeed, for such types of transistors, it is not possible any more to neglect the capacity of the junctions which, although relatively low values, have at the high frequencies rather low impedances. This impedance can influence in a considerable way resistances of entry and exit of the transistor.
In addition to the valid parameters low frequency, it is necessary to take account of the capacities of junctions, i.e. exit and input capacitances, which changes to six the total number of the parameters.
As it is seen figure 10-a, in the case of the assembly bases common, the input capacitance coincides with the capacity of the junction transmitter-bases and that of exit with the capacity of the junction collector-bases.
In the case of the common transmitting assembly, the input capacitance also coincides with the capacity of the junction transmitter-bases while that of exit, more complex, is due to the two other junctions.
The impedances of entry and exit of the transistor are thus given by the parallelization of a resistance r and reactance presented by the capacity C with the frequency of operation considered.
Since these two elements are in parallel, it is advisable to consider the admittances of them rather than the impedances in order to carry out simply the sum of the conductance g = 1 / r, due to the resistive component and of susceptance b = 2 x n x f x C of the capacitive component.
Generally thus, instead of giving the admittance Y, the manufacturer indicates the values of the components g and b for various frequencies. One defers b on the vertical axis and g on the horizontal axis of a diagram in which each point corresponds to a given value of the frequency. An example of diagram relating to transistor BF 195, used in the circuits at high frequencies and intermediate frequencies of the radio operator receivers, is given on figure 11.
Some values of frequency were indicated: 0,45 MHz and 10,7 MHz respectively represent the intermediate frequencies used in the radio operator receivers for the amplitude modulation and the frequency modulation ; 100 MHz represents one of the frequencies of possible emission in frequency modulation (frequency band ranging between 87 and 108 MHz).
With regard to the reaction and coefficients of performance, they are defined here as being the relationship between the variations of the collector current (or basic) and those of the basic tension (or of collector) ; they are also admittances.
The coefficient of
performance Yfe is given by the relationship between variation 
IC
of the collector current and variation VBE
of the corresponding basic tension, is :
Yfe
= IC
/
VBE
This parameter is generally expressed in mS (millisiemens), IC being expressed in mA and VBE out of V (or IC in µA and VBE in mV).
The
coefficient of Yre reaction is on the other hand defined like the
report/ratio of the variation of basic current IB
and the variation of the tension of collector which causes it, is :
Yre
= IB
/ VCE
This coefficient is generally expressed in µS (microsiemens), i.e. the relationship between µA and V.
Since the variations of the current
and the tension are not in phase (because of the internal capacities of the
transistor), it is necessary to specify this dephasing which we call 
; thus, an angle fe
will correspond to the parameter Yfe and an
angle re
will correspond to the Yre parameter.
The values of Yfe,
fe,
Yre and re
vary with the frequency ; they are thus
obtained starting from graphs such as those of the figure 12-b and the figure
12-a.
In these graphs carried out in polar co-ordinates,
one reads the fe values according
to the angular value indicated on the circumference and the module of Y
on one of the rays.
It should finally be noted that these parameters called parameters Y (since Y is the symbol of the admittance) also depend on the collector current of the transistor ; it is thus necessary to have various curves, each one of them corresponding to a given value of IC.
As example, if one wishes to know the parameters Y of transistor BF 195 when it works with a collector current of 1 mA at the frequency of 10,7 MHz, it is enough to consider on various graphics of figure 11 and figure 12 the points indicated by 10,7 MHz on the curves marked by 1 mA.
The following values thus are obtained :
re
= 270° (figure 12-b),
fe
= 352° (figure 12-b).These parameters are useful for calculation of the profit and the impedances of entry and exit of the amplifiers HF like for the checking of their stability.
The next lesson of the semiconductors N° 7 will treat various semiconductors such as the transistors double-base diode, thyristors, diac and of the triac. Then, we will begin the new lessons of electronic digital without forgetting the PRACTICE
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